Quartz

For other uses, see Quartz (disambiguation).

Quartz
General
Category	Mineral
Chemical formula (or Composition)	Silica (silicon dioxide, SiO2)
Identification
Color	Clear (if no impurities); also see Varieties
Crystal habit	6-sided prism ending in 6-sided pyramid (typical)
Crystal system	Hexagonal
Cleavage	None
Fracture	Conchoidal
Mohs Scale hardness	7 - lower in impure varieties
Luster	Vitreous/glossy
Refractive index	1.544-1.553 - DR +0.009 (B-G interval)
Pleochroism	None
Streak	White
Specific gravity	2.65 constant; variable in impure varieties
Melting point	1650 (±75) °C
Boiling point	2230 °C
Solubility	H2O insoluble
Major varieties
Chalcedony	Any cryptocrystalline quartz, although generally only used for white or lightly coloured material. Otherwise more specific names are used.
Agate	Banded Chalcedony, translucent
Onyx	Agate where the bands are straight, parallel and consistent in size.
Jasper	Opaque chalcedony, impure
Aventurine	Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's eye	Fibrous quartz, exhibiting chatoyancy.
Rock Crystal	Clear, colourless
Ruby Quartz	Crimson glass-like crystal, absorbs vast amounts of solar energy
Amethyst	Purple, transparent
Citrine	Yellow to reddish orange, greenish yellow
Prasiolite	Mint green, transparent
Rose quartz	Pink, translucent, may display diasterism
Milk quartz, or snow quartz	White, translucent to opaque, may display diasterism
Smoky quartz	Brown, transparent
Morion	Dark-brown, opaque
Carnelian	Reddish orange chalcedony, translucent

Quartz is one of the most common minerals in the Earth's continental crust. It is made of crystallized silica (silicon dioxide, SiO₂) and can occur in a wide range of varieties. The typical outward shape of the crystal ("crystal habit") is a six-sided prism that ends in six-sided pyramids. This shape, however, may be masked by the crystal's size and natural distortions.

History

As one of the world's most common crustal minerals, quartz has been known to humanity through most of history and has been called by a bewildering array of names. The term "quartz" comes from the German "Quarz," which is of Slavic origin (Czech miners called it křem). Some sources, however, trace the word to the Saxon term "Querkluftertz," meaning cross-vein ore.

Roman naturalist Pliny the Elder (CE 23–79) believed quartz to be permanently frozen ice. In support of this idea, he said that quartz is found near glaciers in the Alps and that large quartz crystals were fashioned into spheres to cool the hands. He was aware of the ability of quartz to split light into a spectrum.

The study of quartz by Nicolas Steno (1638–1686) paved the way for modern crystallography. He discovered that no matter how distorted a quartz crystal, the long prism faces always made a perfect 60-degree angle.

Charles B. Sawyer invented the commercial quartz crystal manufacturing process in Cleveland, Ohio. This process initiated the transition from mined to manufactured quartz.

Occurrence

Quartz occurs in hydrothermal veins (veins produced through hot water circulation) and pegmatites (coarse-grained igneous rocks). Well-formed crystals may reach several meters in length and weigh hundreds of kilograms. These veins may bear precious metals such as gold or silver, and form the quartz ores sought through mining. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals."

Quartz is a common constituent of granite, sandstone, limestone, and many other igneous, sedimentary, and metamorphic rocks.

Tridymite and cristobalite are high-temperature polymorphs of silica (SiO₂), occurring in silica-rich volcanic rocks. Lechatelierite is an amorphous silica glass, which is formed by lightning strikes in quartz sand.

Varieties

Quartz comes in many varieties, including amethyst, carnelian, chrysoprase, flint, onyx, and sardonyx. Some types of quartz have a "macrocrystalline" structure, in which individual crystals are visible to the unaided eye, while other types of quartz have a "microcrystalline" or "cryptocrystalline" structure, in which aggregates of crystals are visible only under high magnification. The cryptocrystalline varieties are translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a generic term for cryptocrystalline quartz.

Milk quartz.

Historically, many of the varietal names arose from the mineral's color. Current scientific naming schemes, however, place greater emphasis on the mineral's microstructure.

Not all varieties of quartz are naturally occurring. Prasiolite, an olive-colored material, is produced by heat treatment, but natural prasiolite has been obeserved in Lower Silesia in Poland. Citrine occurs naturally, but the majority is produced by heat-treatment of amethyst. Also, carnelian is widely heat-treated to deepen its color.

Natural quartz is often "twinned"—that is, two crystals intergrow and share some of their lattice points. For this reason, much of the quartz used in industry is synthetically produced, by what is called a hydrothermal process. This process generates large, flawless, untwinned crystals.

Notable characteristics

Quartz has a hexagonal crystal structure made of trigonal crystals of silicon dioxide (SiO₂, or silica). The typical outward shape of the crystal ("crystal habit") is a six-sided prism that ends in six-sided pyramids. The crystals, however, are often twinned, distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally, the crystals may take the form of a bed, particularly for varieties such as amethyst, in which the crystals grow up from a matrix and only one termination pyramid appears. A quartz geode consists of a hollow rock (usually with a roughly spherical shape) containing a core lined with a bed of crystals.

The density of quartz is 2.65 g/cm³. Its hardness (resistance to scratching) is rated as 7 on the Mohs scale of 1 to 10 (where 10 is the maximum value, assigned to diamond).

Piezoelectricity

A quartz crystal has a property known as piezoelectricity—which means that when mechanical stress is put on the crystal, or when the crystal is made to vibrate, it produces electricity. One of the earliest uses for this property of quartz was in making the pickup for a phonograph.

Today, one of the most ubiquitous piezoelectric uses of quartz is for "crystal oscillators," which are often simply called "quartzes." A quartz crystal oscillator is an electronic circuit in which the mechanical resonance of a vibrating quartz crystal creates an electrical signal with a precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters. The crystal used therein is sometimes called a "timing crystal".

The piezoelectric principle is also used in the quartz crystal microbalance, which can make highly accurate measurements of tiny changes in mass.

Fused quartz

Fused quartz, or fused silica, refers to types of glass containing primarily silica in amorphous (non-crystalline) form. Different forms are produced by varying the manufacturing process. A transparent form is made by melting pure, naturally occurring quartz crystals at around 2000°C in an electric or gas-fueled furnace. Translucent and opaque forms are produced by melting high-purity silica sand in an electric furnace. The opacity is caused by tiny air bubbles trapped in the material.

Synthetic fused silica is made from a silica-rich chemical precursor, resulting in a transparent glass with an ultra-high purity and improved optical transmission in the deep ultraviolet. This material was originally called fumed silica because the high purity manufacturing process involves chemical gassification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust.

The optical and thermal properties are superior to those of other types of glass due to its purity (or rather, its lack of impurities). For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. It has better ultraviolet transmission than most other glasses, and so is used to make lenses and other optics for the ultraviolet spectrum. Its low coefficient of thermal expansion also makes it a useful material for precision mirror substrates.

Chemistry

Fused quartz is a noncrystalline form of silicon dioxide (SiO₂), which is also called silica. (The crystalline form of this material is quartz).

Applications

Specially prepared fused silica is also the key starting material used to make optical fiber for telecommunications.

Because of its strength and high melting point (compared to ordinary glass), fused silica is used as the envelope of halogen lamps, which must operate at a high envelope temperature to achieve their combination of high brightness and long life.

The combination of strength, thermal stability, and UV transparency makes it an excellent substrate for projection masks for photolithography.

Fused quartz has nearly ideal properties for fabricating first surface mirrors such as those used in telescopes. The material behaves in a predictable way and allows the optical fabricator to put a very smooth polish onto the surface and produce the desired figure with fewer testing iterations.

Translucent fused silica is used to make crucibles, trays and other containers for use in high temperature thermal processing, which are chemically inert to to most elements and compounds including virtually all acids, regardless of concentration. Translucent tubes are commonly used to sheath electric elements in room heaters, industrial furnaces and other similar applications.

Physical Properties

The extremely low coefficient of thermal expansion accounts for its remarkable ability to undergo large, rapid temperature changes without cracking.

"UV grade" synthetic fused silica (sold under the tradename Suprasil) has a very low metallic impurity content making it transparent deeper into the ultraviolet. An optic with a thickness of 1cm will have a transmittance of about 50% at a wavelength of 170 nm, which drops to only a few percent at 160 nm. However, its infrared transmission is limited by strong water absorptions at 2.2 μm and 2.7 μm.

"IR grade" fused quartz (tradename Infrasil and others) which is electrically fused, has a greater presence of metallic impurities, limiting its UV transmittance wavelength to around 250 nm, but a much lower water content, leading to excellent infrared transmission up to 3.6 μm wavelength. All grades of transparent fused quartz/fused silica have near-identical physical properties.

The water content (and therefore infrared transmission of fused quartz and fused silica) is determined by the manufacturing process. Flame fused material always has a higher water content due to the combination of the hydrocarbons and oxygen fuelling the furnace forming hydroxyl [OH] within the material. An IR grade material typically has an [OH] content of <10 parts per million.

See also

• Fused quartz
• List of minerals
• Shocked quartz

References

ISBN links support NWE through referral fees

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
• Quartz group - Mineral Galleries
• Quartz - Mineral Galleries
• Quartz - Mineral.net
• Arkansas quartz, Rockhounding Arkansas
• Gilbert Hart Nomenclature of Silica, American Mineralogist, Volume 12, pages 383-395, 1927
• Mindat.org
• Queensland University of Technology Origin of the word quartz.
• PDF of Charles Sawyer's cultured quartz process description

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